Hydraulic circuit for linearly driving a movable roller-holder slider of a pipe bending machine

ABSTRACT

A hydraulic circuit for linearly driving a movable roller-holder slider of a pipe bending machine, comprising a hydraulic cylinder ( 5 ) whose piston rod is connected to a slider holding a movable roller, the cylinder being fed with pressurized fluid from a reservoir ( 16 ) by a pump ( 15 ), through a three-position four-way valve ( 18 ), a check valve ( 12, 13 ) and an interposed throttling valve ( 19 ), that is operated to generate an increased pressure in said low pressure chamber of the hydraulic cylinder ( 5 ) in order to slow down the slider holding the upper roller in its primary motion when a programmable interval is reached from said predetermined position for each working pass.

FIELD OF THE INVENTION

This invention relates to a hydraulic circuit for linearly driving amovable roller-holder slider of a pipe bending machine. The pipe bendingmachine can be of any kind, both symmetrical or asymmetrical. Forsimplicity and clarity sake a pyramidal, symmetrical pipe bendingmachine is referred to below.

BACKGROUND OF THE INVENTION

An upper roller among three rollers of an existing pyramidal,symmetrical pipe bending machine is generally mounted on a slider thatis vertically movable by a hydraulic cylinder. The hydraulic circuitensuring that the movable roller-holder slider is linearly drivencomprises a hydraulic cylinder whose rod is connected to theroller-holder slider. The hydraulic cylinder has an upper chamber and alower chamber, both chambers communicating with respective ducts ofpressurized fluid that is feed from a reservoir by a pump. Athree-position four-way valve and a check valve operate on both ducts.These valves, as well as the pump, are controlled by an electroniccontrol unit.

The roller-holder slider travels downward in a primary motion to abending position and upward in a return motion to a rest position.

As known, pipes or other section bars are bent among the three rollersof a pipe bending machine through an operation including one pass ormore with a result of the desired deformation. In order to obtain thisdeformation, specially when one workpiece or more has to be bent exactlywith the same bending radius, one must keep for every workpiece the sameposition of the movable roller with respect to the fixed rollers,wherein the bending operation is performed.

The valves in the hydraulic circuit, controlled by the electroniccontrol unit, do not ensure an exact positioning of the hydrauliccylinder rod at the required vertical position in the various passes.This is owing to a number of forces in action, among which thefrictional force in the downward motion, the resistance of the materialof the workpiece to be bent, the active forces of the piston of thehydraulic cylinder, the seal deflecting force, the elastic force of thepiston ring, the counteracting differential thermodynamic force, thetime constant of the control response of the electromagnetic valve, theviscosity of the hydraulic fluid and its non homogeneity due to thepresence of air having a coefficient of compressibility different fromthat one of the hydraulic fluid.

In order to remedy the imprecision of the actual point of halting themovable roller-holder slider, mechanical stops have been used in thepast that can ensure, without ascertainable error, the exact position ofthe movable roller in which a respective bending pass has to beperformed.

However, to set the points of halting by mechanical stops is atroublesome operation, that involves an added effort and waste of time,in particular when deformations with great radiuses of curvature have tobe performed.

Further, in the past, in order to solve the problem, complicatehydraulic circuits provided with proportional valves involving increasedcosts have been used.

OBJECT OF THE INVENTION

The present invention aims to overcome the drawbacks above mentioned.

In particular, an object of the present invention is to allow a pipebending machine to operate, determining with precision the position ofpipe bending while a position of bending can be set without requiring amechanical stop device.

SUMMARY OF THE INVENTION

Therefore, the present invention provides a hydraulic circuit forlinearly driving a movable roller-holder slider of a pipe bendingmachine, comprising a hydraulic cylinder whose piston rod is connectedto a slider holding a movable roller that travels in its primary motionto a predetermined position for each pass of one or more passes ofworking operation of a workpiece to be bent and in its return motion toa rest position, the hydraulic cylinder having a high pressure chamberand a low pressure chamber, both chambers communicating with respectiveducts of pressurized fluid fed from a reservoir by a pump, ducts onwhich a three-position four-way valve and a check valve operate, furthercomprising, between said valves, a throttling valve, that is operated byan electromagnet to generate an increased pressure in said low pressurechamber of the hydraulic cylinder in order to slow down the sliderholding the upper roller in its primary motion when a programmableinterval is reached from said predetermined position for each workingpass.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be now described with reference to its preferredembodiment, although it has to be understood that modifications can bemade to the invention without departing from the spirit thereof,referring to the figures of the accompanying drawing, in which:

FIG. 1 shows a diagrammatic side view of a partially opened pipe bendingmachine, to which a hydraulic circuit according to the invention isapplied;

FIG. 2 shows a diagram of hydraulic circuit according to the invention;and

FIGS. 3 to 6 show different operative positions of two valves of thehydraulic circuit according to the invention in its operation on thepipe bending machine.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawing, the general appearance of a pipe bendingmachine, generally denoted as 1, is shown in FIG. 1. The pipe bendingmachine 1 is equipped with a hydraulic circuit according to theinvention.

The pipe bending machine shown by way of example is of a symmetricalpyramidal kind. It has frontally (on the right hand side in FIG. 1) apair of fixed lower rollers (only one roller, denoted as 2, is shown)and an upper roller 3. The upper roller 3 is mounted conventionally on aslider (not shown) that is connected to a piston rod 4 diagrammaticallyrepresented in FIG. 2. The piston rod 4 is a part of a hydrauliccylinder 5 having an upper chamber 6 and a lower chamber 7.

Owing to the motion of the piston rod 4, the slider holding the upperroller 3 is movable downward during a primary motion from a generalposition indicated by an axis g to a predetermined position of axis 1,as shown in an explanatory way in FIG. 1. The bending operation of aworkpiece (not shown) is performed during a travel including one pass ormore. In every pass, said predetermined position of axis l is selectedfor each workpiece. If e.g. it is intended that two equal workpieces tobe bent are worked by two passes, and an equal end position of pipebending, but a different intermediate position is chosen for everyworkpiece to be bent, two workpieces with different dimensionalcharacteristics would be obtained.

One would appreciate the importance that bending positions are achievedexactly as much as possible.

As constructively and diagrammatically shown in FIGS. 1 and 2respectively, the upper chamber 6 and the lower chamber 7 of thehydraulic cylinder 5 are communicating through their ports 8 and 9 withrespective ducts 10 and 11 of pressurized fluid, and a pilot-operated toclose check valve, that consists of a pair of single-acting valve 12 and13, is provided.

A pressurized fluid, in general oil for hydraulic circuits, is fed froma reservoir 14 through a motor-pump unit 15. As best shown in FIG. 2, atleast a filter 16 and a pilot-operated safety valve 17 are provided inthe circuit of the pump. Further conventionally, a three-positionfour-way valve 18 operates on both ducts 10 and 11. The valves, as wellas the pump, are controlled by an electronic control unit (not shown).

According to the invention, a throttling valve 19, that is controlled byan electromagnet, is joined to the valve 18 on the same ducts 10 and 11.

Also the throttling valve 19 is operated by said electronic control unit(not shown) to generate a back pressure in the lower chamber 7 of thehydraulic cylinder 5. In fact, in the primary motion, i.e. in thedownward travel of the movable roller 3, when the predetermined bendingposition which is defined by the axis l of the movable roller isapproaching, it is suitable to slow down the slider holding the upperroller 3 so that the last one can reach exactly the bending position.This deceleration, e.g. from the position of axis h is obtained byoperating, as desired, the throttling valve 19 in order to graduallyslow down the movable roller travelling downward, up to the completeclosure of the valve in the desired end position for the bending passthat is performed.

The interval h-l inside which the slow down is performed is programmableaccording to the desired precision and so on.

This deceleration is obtained through the combined operation of thethree-position four-way valve 18 and the throttling valve 19, as shownin FIGS. 3 to 6.

In these figures a streamline of high pressure fluid coming from thepump unit 15 is indicated as A, and a streamline of low pressure fluidreturning from the hydraulic cylinder 5 operating the slider holding theupper roller 3 is indicated as B.

Referring to FIG. 3, the three-position four-way valve 18 and thethrottling valve 19 are shown in an arrangement in which the highpressure streamline A goes to the high pressure chamber 6 of thecylinder, and the low pressure streamline B indicates the return flowexiting the low pressure chamber 7. The throttling device, denoted as20, of the throttling valve 19 is in a non working arrangement andremains in this position until the predetermined position of axis h ofthe movable roller 3 is reached (FIG. 1). From this position thethrottling device 20 is operated, as shown in FIG. 4. The streamline Bis diverted to a bypass 21, where the rate of flow is reduced.

Consequently, there is a pressure increasing in the low pressure chamber7 of the cylinder 5. As a result, the downward travel of the movableroller 3 is slowed until the position of axis l is reached in which theflow is stopped, and both chambers 6 and 7 are under the same operatingpressure. In the same time, all slacks are taken up, including air ballsthat are the decisive factor of error by having a compressibilitydifferent from that one of the hydraulic fluid. Now, the three-positionfour-way valve 18 and the throttling valve 19 are brought to a restarrangement shown in FIG. 5.

In order to return the movable roller 3 to its former position of axisg, the three-position four-way valve 18 and the throttling valve 19 arebrought to an arrangement shown in FIG. 6 in which the throttling device20 is not working. Now, as for the throttling valve 19, the arrangementof FIG. 3 is repeated, while the three-position four-way valve 18performs the backflow. Through this operation, the chamber 7 of thecylinder 5 becomes the high pressure chamber, while the chamber 6becomes the low pressure chamber.

The throttling valve 18 can be a unidirectional valve. In alternative,the throttling valve 18 can be a bi-directional valve, in order toassure a slow down in both primary and return motions.

The present invention has been described with reference to its specificembodiment, but it would be expressly understood that modifications,addition and/or omissions can be made without departing from the spiritof invention as defined in the enclosed claims.

1. A hydraulic circuit for linearly driving a movable roller-holder slider of a pipe bending machine, comprising a hydraulic cylinder whose piston rod is connected to a slider holding a movable roller that travels in its primary motion to a predetermined position for each pass of one or more passes of working operation of a workpiece to be bent and in its return motion to a rest position, the hydraulic cylinder having a high pressure chamber and a low pressure chamber, both chambers communicating with respective ducts of pressurized fluid fed from a reservoir by a pump, ducts on which a three-position four-way valve and a check valve operate, characterized in that the hydraulic circuit further comprises, between said valves, a throttling valve, that is operated by an electromagnet to generate an increased pressure in said low pressure chamber of the hydraulic cylinder in order to slow down the slider holding the upper roller in its primary motion when a programmable interval is reached from said predetermined position for each working pass. 